Fixes in QueryStringDecoder

Motivation:

QueryStringDecoder has several problems:
- doesn't decode correctly path part with `+` (plus) sign in it,
- doesn't cut a `fragment` (after `#`) from query string (see RFC 3986),
- doesn't work correctly with encoding,
- treat `%%` as a percent character escaping (it's don't described in RFC).

Modifications:

- leave `+` chars in a `path` part of uri string,
- ignore `fragment` part (after `#`),
- correctly work with encoding.
- don't treat `%%` as escaping for the `%`.

Result:

Fixed issues from netty#6745.